It is well known that potassium (K+) can be replaced by sodium (Na+) to a great extent in sugar beet (Beta vulgaris). However, the possible extent of substitution is limited also in sugar beet. It was not clear which process limits the substitution of K+ by Na+ in young sugar beet. It is known that K+ is required for the processes of transpiration, growth, and protein synthesis. The aim was to find out which process is most sensitive due to the substitution of K+ by Na+.
Therefore, sugar beets were grown at various substitution levels (0.25%, 25.00%, 50.00%, 75.00%, 97.50%, 98.75%, and 99.75% substitution). The sum of supplied K+ and Na+ concentrations was kept constantly equal at the level of adequate K+ nutrition of 4 mM. The approach excluded chloride or osmotic effects.
The process of transpiration was not affected by the substitution. The transpiration rate was not increased even when 99.75% of K+ were substituted by Na+. The growth of young sugar beet plants was inhibited under the substitution of 99.75% K+ by Na+. The supply of sugars was inhibited when 98.75% of K+ were substituted by Na+. Protein content per shoot was chosen as a parameter for the quantification of the effect of the substitution on net protein synthesis. Protein contents were reduced when 97.50% of K+ were substituted by Na+. Since transpiration, growth, and the supply of sugars were not inhibited by this substitution level, net protein synthesis was the most sensitive process during substitution of K+ by Na+ in young sugar beet.
The inhibition of net protein synthesis due to a substitution-induced magnesium (Mg2+) deficiency or due to a shortage of proteinogenic amino acids was excluded. Free amino acids even accumulated with increasing the extent of the substitution. Therefore, it was hypothesized that the process of translation itself is inhibited due to the substitution of K+ by Na+.
In order to test this hypothesis, ribosomes were isolated from growing leaves of sugar beet and maize which were grown under control conditions. In an in vitro approach the translation of the ribosomes from both plant species was quantified via the incorporation of 35S-methionine in peptides and proteins. In order to avoid the translation by ribosomes of plastids and mitochondria, organellar ribosomes were inhibited by means of the addition of chloramphenicol to both in vitro systems. Magnesium and K+ concentrations were identified which meet the requirements of both in vitro systems. Potassium was substituted by Na+ at different levels (0%, 20%, 40%, 60%, and 80% substitution). The effect of different substitution levels on the 35S-methionine incorporation of both in vitro systems was determined.
The substitution of K+ by Na+ inhibited the in vitro translation of both plant species. This shows that the process of translation itself is inhibited by the substitution and explains the observed inhibition of net protein synthesis in vivo under substitution conditions. However, the ribosomes of sugar beet exhibited higher level of tolerance than the ribosomes of maize.
Besides the plasma membrane H+-ATPase, the ribosomes represent the second example for increased tolerance of high Na+/K+ ratios in the metabolism of sugar beet. This indicates that the salt resistance of sugar beet is not only based on the successful compartmentation of ions in vacuoles, but also on the Na+ tolerance of key enzymes.